Increased industrial activities and a rapidly escalating increase in demand of fossil fuel for power generation and transportation, has led to increased emission levels which has demanded highest scrutiny with respect to the health hazard and environmental pollution. A key pollutant in hydrocarbon transportation fuels is sulphur which apart from being responsible for formation of harmful sulphur di-oxide emissions during fuel combustion, also causes poisoning of the catalytic converters in automobiles in turn leading to increase in NOx emissions.
New fuel quality standards are being imposed for the control of such emissions through reduction in sulfur content in the hydrocarbon fuels. There are several approaches for such desulfurisation of hydrocarbon fuels. Some of these approaches were particularly examined to achieve ultra deep desulfurisation. The conventional hydro-desulfurisation of hydrocarbon fuels being currently practiced in the petroleum refining industry involves contacting of hydrogen with the hydrocarbon stream in presence of catalyst at elevated temperatures and pressures to convert sulfur compounds present therein to hydrogen sulfide in. This hydro-desulfurisation process is at present unable to achieve ultra low sulphur levels in the fuel due to low reactivity of refractory sulfur species under conventional conditions and also strong inhibition of the reaction by the reaction products H2S, NH3, nitrogen and even aromatic species. The environmental regulations in US and European countries call for a reduction in sulfur content to as low as 10 ppm. To achieve these goals, the operating conditions of hydrodesulfurisation need to be more severe with respect to temperature and pressure and this may lead to increased process cost. Among the new alternative approaches being studied, oxidative desulfurisation (ODS) appears to be particularly promising mainly due to ambient temperature and pressure operating conditions, which permits a significant cost reduction. In such a process, sulfur compounds present in the hydrocarbon fuels are oxidized by an oxidant into sulfones and sulfoxides, which are subsequently removed from the oxidized fuel by taking advantage of their different chemical and physical characteristics. The efficiency and economics of an oxidative desulfurisation process is strongly dependant on the method used for separating the sulfones/sulfoxides from the oxidized fuels.
Several separation methods have been suggested for use in removal of oxidized sulfur from hydrocarbon fuels. These methods use either solvent extraction, catalytic decomposition or adsorption. Reference may be made to Collins et al (Journal of Molecular Catalysis A: Chemical 117, 1977, 397) who reported the removal of sulfones from gas oil by solvent extraction using γ-butyrolactone as solvent or by adsorption on silica gel to obtain ultra low sulfur gas oil. F. Zannikos et al (Fuel Processing Technology, 42, 1995, 35) discussed solvent extraction route using polar solvents such as N-methylpyrrolidone (NMP), methanol, dimethylformamide (DMF) to remove oxidized sulfur from hydrocarbon fuel However he noted that there were significant losses (approx 30%) of usable oil which would not make the process economical despite advantages of low temperature and pressure operation.
U.S. Pat. No. 6,160,193 discloses a process also based on extraction of oxidized sulfur with insoluble polar solvents like dimethylsulfoxide (DMSO) thereby reducing the losses of the usable oil somewhat but losses were still significant. Otsuki et al (Energy and Fuels, 14, 2000, 1232) studied the removal of oxidized sulfur compounds by extraction using N,N dimethylformamide, acetonitrile, methanol, dimethylsulfoxide and sulpholane as solvent. Mei et al (Fuel 82, 2003, 405) reported removal of sulfones from diesel using solvent acetonitrile.
In general one of the major drawbacks of solvent extraction method is the appreciable solubility of hydrocarbon fuels in polar solvents with which leads to significant losses of usable hydrocarbon fuel. Such a loss is completely unacceptable on a commercial basis. Beside this, sulfones are polar compounds and form strong bonding with polar solvents and it is difficult to remove them from the solvents to below 10 ppm. Hence there will be build-up of sulphones in the solvent during solvent recovery for recycle. Moreover the integrated extraction of sulfones by polar solvents also makes the process flow scheme more complicated due to increase in the numbers of columns required in various steps such as extraction, raffinate wash column, solvent recovery column, and extract wash column.
Apart from solvent extraction, use of a large number of commercially available adsorbents has also been reported in the prior art for the selective removal of oxidized sulfur compounds. Reference may be made to U.S. Pat. No. 5,958,224 which discloses a process for removal of sulfones by adsorption on solid adsorbents such as activated carbon, bauxite, clay, coke, alumina or silical gel with pores large enough to adsorb the multi ring oxidation product. However the amount of oil treated per unit weight of adsorbent in this particular process is low and the process was not described with actual hydrocarbon fuels but with model hydrocarbon feed. US patent application 20040007501 discloses a process for the desulfurisation of sulfur containing hydrocarbon fuel by treating it with an oxidizing agent and subsequent removal of oxidized sulfur compounds with a sorbent comprising a promoter metal component and zinc oxide followed by regeneration of sulfur loaded sorbent with an oxygen containing regeneration scheme. The process condition requires very high pressure (35 bar) and high temperature (398° C.) and presence of hydrogen. U.S. Pat. No. 6,368,495 discloses a process for the oxidation of thiophenes and thiophene derivatives present in petroleum fraction to sulfones followed by decomposition of sulfones using catalyst like double layer hydroxides, molecular sieve, inorganic metal oxides or a mixture thereof. The process is carried out at pressure of 7 bar and temperature of around 475° C. A 40 to 74% decrease in sulfur level was achieved after 50 hours time on stream. Ishihara et al (Applied Catalysis A. General 279, 2005, 279) reported oxidation of sulfur compounds present in desulfurised light gas oil with sulfur content 40 ppm followed by removal of the sulfones formed by adsorption over silica gel. A low breakthrough capacity of around 0.2 mg/g was reported. European patent 0565324 A1 mentioned a method for removal of organic sulfones by adsorption on alumina or silica. WO patent 2005019386 discloses an integrated process for deep desulfurization of hydrocarbonaceous fuels by HDS followed by oxidation of the remaining sulfur compounds present and their subsequent removal by adsorption over microporous solids such as silicalite, ZSM-5, zeolite beta, zeolite L, zeolite X and Y. U.S. Pat. No. 6,402,940 discloses a process for desulfurization of fuels such as diesel oil and similar products which involves oxidation of sulfur compounds with a oxidizing solution to sulfones and followed by separation of sulfones by adsorption on alumina with a typical sulfur breakthrough capacity of 2 mg/g which is very low.
In summary the various techniques based on solvent extraction and adsorption with microporous adsorbents as revealed in the prior art for the removal of oxidized sulphur compounds from hydrocarbon fuels suffer from major drawbacks of low product yields and build up of sulphones in the solvent in case of solvent extraction processes and low adsorbent capacity for sulphones in case of adsorption processes. In recent years much attention has been paid to the development of ordered mesoporous oxide based materials due to their large pore sizes and controlled pore size distribution which may be beneficial in allowing accessibility of large molecules to the surface active sites. It is worth mentioning here that these mesoporous materials can be tailor made with respect to their surface and pore characteristics to suit to a large number of applications such as catalysis and adsorption. Feng et al (Science 276, 1997, 923) described an application of functionalized mesoporous material for the efficient removal of mercury. Damian Perez Quintanilla et al (Journal of Materials Chemistry 2006, 16, 1757) showed the use of chemically modified mesoporous silica for adsorption of Cd(II) from aqueous media. Zheng Yan et al (Journal of Materials Chemistry 2006, 16, 1717) reported the application of pyridine functionalized mesoporous silica as an efficient adsorbent for the removal of acid dyestuff. U.S. Pat. No. 6,756,022 describes a two step process for the adsorption of un-oxidized organo-sulphur compounds from fuels using FSM type mesoporous silica materials. This step is followed by in-situ oxidation of adsorbed organo-sulphur compounds to sulfoxides/sulfones which are subsequently desorbed from the mesoporous materials and further recaptured by another adsorbent (active clay or zeolite). There is a large scope for mesoporous material based adsorbents for the selective removal of sulfones and sulfoxides from oxidized hydrocarbon fuels, particularly since their pore sizes can be controlled to ensure accessibility of the large sulphone molecules into the adsorbent. However the prior art does not reveal any use of ordered mesoporous material for adsorption of oxidized sulfur compounds from hydrocarbon fuels.